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1. Field of the Invention
The invention relates to a method of measuring the distortion angle of the weft threads or rows of stitches in a strip of textile, and to an apparatus for implementing this method.
2. Description of Related Art
In view of the fact that the distortion angle of a woven or knitted strip of textile must be measured so that the result of the measurement can be used in machines for the automatic correction of textile strips (tentering or straightening machines), such measurement is a technical problem that has existed for decades and has already been solved in a more or less costly and reliable way.
The German patent DE-PS 674 750 describes a control mechanism for the automatic straightening of woven, light-transmitting cloth strips for web-tentering or similar machines, which derives its input signals from an arrangement of photoelectric cells that are illuminated by light passing through narrow slits, which are oriented at an angle to the transport direction of the cloth strip, and then through the moving web. By comparison of the signals from photocells associated with two slits that are perpendicular to one another, as a function of time, a control signal is obtained for tentering chains that have a straightening action on the web, although this arrangement does not explicitly measure the distortion angle.
In the patent DE-OS 1 635 266 a method of determining the weft-thread position of moving strips of woven material is described, along with an apparatus for implementing this method, in which a photoelectric detection device is likewise employed. The apparatus described here also, in one embodiment, comprises slots arranged in a V shape, with each of which are associated two photocells that are illuminated through the web. In another embodiment the measurement device comprises one photocell and one slot which, together with the photocell, can be moved back and forth like a pendulum, so that when the time-dependence of the photocell signal is evaluated in synchrony with a signal indicating position of the slot, information comparable to that obtained with the two-slot arrangement is provided. Disadvantages in a certain sense are the inertia of the mechanically moved arrangement and its mechanical wear and tear.
In the patent EP-B 0 291 729 another method and an apparatus for measuring the weft-thread or stitch-row position of strips of textile are described, in which by means of a photoelectric transducer a slit-like section is observed; in this case the transducer is formed by an arrangement of CCD arrays with a plurality of sensor fields (to be read out in series). In this printed specification several variants of concrete arrangements having one or two such CCD-sensor arrays are described, as well as a number of variants for evaluating the sensor signals. The evaluation principle is that as the textile strip moves past, the weft threads (which appear as dark fields) and the intervening spaces (which appear light) continuously illuminate the individual sensor elements of the CCD array(s) more or less strongly in alternation, and the corresponding light-dark signal sequence is analyzed digitally by a threshold-value discrimination. The algorithm for this analysis is of course chosen in accordance with whether one or two sensor arrays are employed and how these are positioned relative to the transport direction. In this solution the information obtainable from the results of the evaluation depends substantially on the specific design of the sensor arrays chosen, and is to some extent satisfactory only if two sensor arrays are used; however, there is need for improvement with respect to the accuracy of the results and the flexibility of the arrangement for differentiated evaluations.
It is thus the object of the invention to disclose a method of this generic kind and an apparatus for implementing this method that is distinguished by high accuracy and reliability as well as versatility with respect to various evaluation tasks.
This object is achieved in regard to its methodological aspect by a method briefly described as follows. A method of measuring a distortion angle between the longitudinal extent of the weft threads or rows of stitches in a textile strip (T) that is being continuously transported in a transporting apparatus (M) and the perpendicular to the transport direction, by means of a lighting arrangement (100A, 200A), a photoelectric sensor arrangement (FIG. 1; FIG. 2; 107) with a plurality of sensor elements that are interrogated individually to obtain discrete light-dark sampling values, and an evaluation device (115) for processing the values sampled from the sensor fields in order to obtain the distortion angle is performed by using as a sensor arrangement an arrangement (FIG. 1; FIG. 2) of sensor elements with a plurality of rows and columns in which individual elements can be accessed as desired. In a sequence of scanning steps different predetermined groups of sensor elements are interrogated, each of which is situated substantially on a sensor-field line (FIG. 1; FIG. 2). The groups involved in each scanning step are determined such that the scanned sensor-field lines, which in particular have one sensor field in common, are positioned at progressively altered angles with respect to the transport direction. With regard to the implementation aspect, the invention is achieved by an apparatus briefly described as follows. The apparatus comprises a lighting arrangement (100A, 200A), a photoelectric sensor arrangement (FIG. 1; FIG. 2; 107) composed of sensor elements, each accessible individually as desired, disposed in a plurality of rows and columns, a scanning-control device (113) to define the predetermined sensor-field line at each scanning step, and an evaluation device (115) connected to the sensor arrangement for processing the values sampled from the sensor elements in order to determine the distortion angle.
The invention includes the essential idea of producing an electronically moved slot, so to speak, linked to the fundamentally well-established principle of the moving slot and the use of a photoelectric sensor arrangement with a plurality of sensor fields. It further includes the idea of using for this purpose a field arrangement, driven in a suitable manner, that consists of optionally selectable photoelectric sensor elements with a plurality of arrays and slots. The field arrangement is controlled in such a way that in each detection step of the measurement procedure a linear array of sensor elements (in the following termed briefly xe2x80x9csensor-field linexe2x80x9d) is scanned and in each detection step the scanning configuration is changed in such a way that in comparison to the preceding step a (virtual) rotation of the sensor-field line occurs.
By this means, in one scanning cycle over a predetermined angular range a xe2x80x9crotating sensor arrayxe2x80x9d is realized, which imitates a swingable detection slot but has no inertia and is not subject to wear; furthermore, depending on the number of sensor elements in the arrangement and the resulting resolution of the apparatus, it allows extensive additional possibilities for analysis. The electronically achieved range of rotation angles covered in a scanning and analysis cycle is selected in dependence on the particular conditions for a specific form of use, as is the (virtual) rotation-angle increment between the individual scanning steps, and is maximally 180xc2x0.
The above-mentioned (virtual) rotation of a sensor-field line, which is realized by suitable choice of the sensor elements within a two-dimensional sensor array that are to be scanned in consecutive steps, can also take the form of xe2x80x9coscillatingxe2x80x9d about an intermediate orientation of the sensor-field line formed by the selected sensor fields. In this case, in particular when reference is made to previously measured valuesxe2x80x94that is, taking into account a distortion that has already formed in the textile stripxe2x80x94the further development of this distortion can be followed more accurately in an advantageous way. That is, when by this means in consecutive sampling or scanning steps no relatively large angular increments are detected, but rather small changes in the vicinity of the distortion angle that had formed in the pastxe2x80x94which naturally functions particularly well when the arrangement comprises very many sensor fieldsxe2x80x94the accuracy of the measurement can be enhanced.
Even in the case of such a design, however, with sensor-field lines that (virtually) oscillate about an intermediate orientation, in a further embodiment it is useful after a certain number of steps to insert a scanning step of a sensor-field line defined by a larger angular increment, so as not to lose sight of the actual orientation of the weft threads or rows of stitches if there are sudden changes in the distortion angle.
The method outlined here can be implemented with no difficulty by a microprocessor-controlled measurement arrangement with corresponding storage possibilities for previously measured values. It is also possible to select among various stored program sequences, in which series of scanning steps with orientations of the scanned sensor-field line that vary by small angular amounts (rotating or oscillating) alternate with virtual rotations by larger angular amounts.
In a preferred embodiment the sensor arrangement is a matrix arrangement in rectangular or square form, and the scanned sensor-field lines are determined by actuating this matrix arrangement in such a way that all sensor-field lines are of substantially the same length. Therefore during their electronic rotation within the overall arrangement in the course of a scanning cycle they substantiallyxe2x80x94depending on the choice of virtual pivot pointxe2x80x94describe a sector of a circle or two diametrically opposed sectors (maximally a complete circle). In view of this, it is of course also possible to employ a circular sensor arrangement at the outset, but in the normal case this option would not be chosen because of the cost advantages of square or rectangular sensor arrays, which are manufactured in large numbers.
To monitor the distortion angle of a broad strip of material, and in particular also to detect garland-like distortion, it is advantageous to use a plurality of sensor arrangements distributed over the width of the textile strip, which are scanned synchronously with one another in the manner outlined above, and the output signals of which are evaluated in connection with one another in order to obtain a full-width distortion profile for the textile strip.
The method proposed here and the associated apparatus, because of their high resolution and the very short cycle time for the individual detection steps, make it possible to insert additional detection steps between consecutive scanning cycles (or, where appropriate, between individual scanning steps within a cycle). Such additional steps serve in particular as a means of obtaining a complete picture of the textile strip or a picture that reveals the thread density by counting the number of passing threads, or in order to monitor processes of stretching or shrinking. These possibilities emphatically document the extraordinary versatility of the proposed solution.
In an embodiment preferred with respect to the performance parameters and cost, the photoelectric sensor arrangement is a CMOS array (in particular in the form of a matrix) in which the sensor fields are formed by photodiodes and which preferably comprises 512xc3x97512 or more sensor fields (pixels). Such a number of sensor fields enables an alternative approximation of linear sensor groups in a large number of angular positions relative to the transport direction. In addition it enables an informative, complete picture to be obtained for further evaluation tasks.
The lighting arrangement belonging to the overall apparatus, which preferably operates by transmitted light, is in particular designed as an infrared lighting arrangement. Implementation in the form of an LED array (in particular likewise matrix-shaped) is advantageous in comparison to the customarily used arrangements of incandescent bulbs in that it enables practically inertia-free control and can achieve a distinctly longer working life. If in a particularly advantageous design the LEDs of the array are so constructed that their emission behavior is matched to the sensitivity curve of the sensor elements, in particular the photodiodes of the CMOS array, in addition considerable savings in energy can be achieved.
The lighting arrangement is advantageously housed all together in a lighting head that can be called a xe2x80x9cspotlightxe2x80x9d, the dimensions and positioning of which are designed to suit the sensor field disposed on the opposite side of the textile strip, which can be housed in a scanning or measurement head. The measurement head and the lighting head, or a plurality of such heads that can be employed together, are each provided with a mechanism to adjust the head position or to guide the head. In addition the measurement heads are provided with an evaluation unitxe2x80x94advantageously contained in a separate housingxe2x80x94to collect and evaluate the measured data. This unit comprises a suitable interface for incorporation into the system, for example an ethernet interface, and can function as server in a TCP-/IP network for clients such as one or more PCs with which to operate the arrangement and display the results graphically, a straightening machine to make use of the results of the measurements, a master computer or the like. A network module consisting of several measurment and lighting heads then provides a complete distortion profile of the textile strip, which can be graphically displayed by the above-mentioned PC and where appropriate either processed further or used directly, by way of a straightening-machine module, to control a straightening machine to eliminate the distortion in the textile strip. Also, however, the PC or PCs can be used to influence the control of the measurement process and the evaluation of the measured data, for example by selecting different angular positions or rotation-angle increments of the sensor-field lines or choosing particular filter algorithms in the evaluation of the results.
Additional advantages and useful features of the invention will be apparent from the following description of preferred exemplary embodiments with reference to the figures.